1. Field of the Invention
The invention relates to a method and an arrangement for managing the transfer of packet data in a cellular system. The invention is advantageously applied in WCDMA (Wideband Code Division Multiple Access) based mobile communications systems, such as UMTS (Universal Mobile Telecommunications System), which uses TDD (Time Division Duplex) and FDD (Frequency Division Duplex) modes.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
To aid in understanding the invention, it is below described in more detail the structure of the UMTS system, which is a so-called third-generation telecommunications system, and the transfer of packet data in the UMTS. It should be noted, however, that the channels, protocol layers and signaling procedures presented here are just examples associated with the UMTS system and the application of the present invention is in no way limited to them. Methods according to the prior art are also described in the patent application document WO 96/37079 and in the proposal for a standard ETSI, European Telecommunications Standards Institute; UMTS YY.01 UE-UTRAN Radio Interface Protocol Architecture; Stage 2, 18.12.1998, and UMTS (YY.02) Layer 1; General Requirements, 12/1998.
FIG. 1 schematically shows a third-generation digital cellular system. The entity comprised of base station subsystems (BSS), including radio network controllers (RNC) and base stations (BS), is called a UTRAN (UMTS Terrestrial Radio Access Network). Core networks CN comprise different exchange systems, such as MSC (Mobile Services Switching Center) and SGSN (Serving GPRS Support Node; GPRS=General Packet Radio Service), which in addition to versatile communications facilities may also offer various intelligent network services. A mobile station MS can be connected with a plurality of core networks CN via UTRAN.
FIG. 2 shows protocol layers in a proposed UMTS system. The lowest protocol layer between the mobile station MS and radio access network UTRAN is Layer 1 (L1) 200, 201, which corresponds to the physical radio link. Above that layer there is an entity corresponding to layer 2 of the conventional OSI model, with a media access control (MAC) layer 202, 203 at the bottom and above it a radio link control (RLC) layer 204, 205. On the top there is a radio resource control (RRC) layer 206, 207 of layer 3 of the OSI model. In between the radio access network UTRAN and the interworking unit of the core network IWU/CN there is a so-called Iu interface where OSI model layers L1 and L2 (blocks 208 and 209 in FIG. 2) correspond to the above-described layers from L1 to RLC, and OSI model layer L3 (RANAP; blocks 210 and 211 in FIG. 2) corresponds to the above-described RRC layer. The IWU is needed if the UTRAN is connected to a second-generation core network, but if there is a third-generation MSC or SGSN the IWU is not needed but the Iu interface suffices.
In the radio interface of the UMTS system, user data and signaling may be sent either on a dedicated channel (DCH) allocated to the mobile station (for a given service offered to the mobile station) or on a common channel. Common channels include e.g. the random access channel RACH, forward link access channel FACH, broadcast channel BCH and the paging channel PCH.
The RACH is used only in the uplink direction. As the RACH is not reserved there is a risk that multiple mobile stations will be using it simultaneously so that a collision occurs on the radio path and the data sent cannot be received. When using the RACH, the identifier of the mobile station originating the transfer has to be sent as well.
Burst transmission power on the RACH is determined using open loop power control. Prior to the transmission of a random access burst the mobile station measures the received power on the downlink primary common control physical channel (CCPCH). In addition, the system informs the mobile station, on the BCH channel, about the transmission power of the CCPCH channel in question. In addition to these data, the transmission power determination uses the uplink interference level information as well as information about the required signal-to-interference ratio (SIR), which are sent to the mobile station on the BCH.
When transferring user data or signaling over the FACH, the identifier of the target mobile station has to be sent as well. Slow, but not fast, transmission power control may be used on the FACH. The transfer rate may be changed on a short notice on the FACH.
A downlink BCH uses a fixed transfer rate. On the BCH, a transmission always covers the whole cell. Likewise, on a downlink PCH a transmission always covers the whole cell.
A dedicated channel DCH may be reserved in the downlink and/or uplink direction. Fast transfer rate changes are possible on the DCH. Also associated with the DCH is fast transmission power control.
Moreover, it is possible that the channel allocated to the mobile station be available to other mobile stations as well, being then a so-called shared channel. Below, a dedicated channel may also refer to such a channel.
The MAC adapts the logical channels and the transfer channels mentioned above in such a manner that the broadcast control channel (BCCH) is transferred on the BCH, the paging control channel (PCCH) is transferred on the PCH, and the dedicated control channel (DCCH) and dedicated traffic channel (DTCH) are transferred on the DCH. In addition, the DCCH and DTCH may also use the FACH/RACH as well as a downlink shared channel (DSCH).
Downlink packet data transfer may be performed in three different ways:                Packet data are transferred on the FACH which the mobile station is currently listening to.        Packet data are transferred on another FACH than the one that the mobile station is listening to, so that the UTRAN first assigns the mobile station a new FACH for the transfer.        Packet data are transferred on a DCH.        
Assignment of a DCH channel is signaled on the current FACH. The UTRAN then uses the downlink DCH to transfer the data packets and control information. The DCH may also be assigned using a special signaling DCH (so-called Control Only mode). The mobile station uses the uplink DCH to transfer acknowledgments and control information.
For downlink data the decision to use a common or a dedicated channel may be network implementation dependent, in which case no radio interface signaling is needed.
Uplink packet transfer may be performed in one of two alternative ways:                Data are transferred on the RACH and acknowledgments on the FACH, or        The RACH/FACH is used to assign a DCH for data transfer. The DCH may also be assigned using a special signaling DCH.        
FIG. 3 illustrates packet data transfer on an uplink common channel. A random access burst 301 sent on the RACH is accompanied by a user data packet 302. Such user data packets may be sent on arbitrary moments. The data packets can be acknowledged on the downlink FACH (not shown in FIG. 3).
A DCH is allocated to the mobile station either for a given period of time or indefinitely, in which case the network, upon noticing that the mobile station has stopped transmitting, orders it to release the channel.
FIG. 4 illustrates data packet transfer on an uplink dedicated channel DCH allocated to the mobile station for a certain period of time T1. A DCH request is made using a random access burst 404 sent on the RACH. The random access burst may also include information about the size of the data packet to be sent or the transfer rate required by the mobile station. If the transfer rate is included as a parameter, the mobile station may select it from among transfer rates allowed for the radio bearer in question. The DCH is allocated using a burst 405 sent on the FACH. The data packets 406–408 are then sent on the DCH 420 according to the allocation made by the system. As the time period T1 reserved for the mobile station comes to an end, the channel is released unless the mobile station requests for additional capacity prior to the end of the time period. This request for additional capacity may be made using a message sent on the DCH.
FIG. 5 illustrates data transfer on a dedicated channel DCH allocated for an indefinite period of time. Allocation of the DCH 520 is requested using a burst 511 sent on the RACH, acknowledged 512 by the system by allocating the DCH. The mobile station may then send data packets 506–508 on the DCH allocated to it. Upon detecting a pause T2 of sufficient duration following a transmitted data packet the system sends to the mobile station a channel release command 513 in order to free the channel.
The problem with the uplink packet data transfer is that the system has no information about the packets to be sent on which to base its channel selection decision. Thus the information about the data packets to be transferred would have to be sent to the system, whereafter the system would have to send the information about the decision on the use of a common vs. dedicated channel to the mobile station. This information transfer uses up traffic capacity and slows down the transfer of packet data.
An object of the present invention is to provide a solution for the management of uplink packet data, avoiding the aforementioned problem relating to the prior art.
An idea of the invention is that the decision about the channel to be used for the packet data transfer is made dependent on a channel selection parameter, and parameters needed in the decision-making are sent to the mobile station. The parameters are advantageously sent on a common channel such as the BCH, FACH or PCH. The parameters may also be sent on a DCH if one is allocated to the mobile station. Parameters to be sent advantageously include the maximum packet size allowed for the RACH, current RACH load, etc. The parameters may concern all mobile stations, a subset of the mobile stations or one mobile station in the area in which the parameters are sent. The invention decreases the signaling load associated with the allocation of packet data transfer capacity as well as minimizes the delay associated with the starting of data transfer.
The decision about whether to use a common or a dedicated channel may be based on a plurality of channel selection parameters such as:                size of data packet; amount of data in RLC buffers or information obtained from higher layers about the amount of data to be transferred,        bit rate required,        allowable transfer delay,        priority or importance of the data to be transferred,        channel load,        power level required for the transfer on the RACH, and        maximum packet size transferable on the RACH.        
The mobile station usually has the information on the bit rate required, allowable transfer delay and the priority of the data to be transferred for the uplink packet data transfer. The use of these parameters in the channel selection is essential to achieve a sufficient data transfer quality level.
It is important to provide the mobile station with the common channel (RACH) load information in order to achieve the desired reliability of transfer, transfer rate and transfer delay on the channel selected, and to make it possible to determine the transmission power for the random access burst.
The maximum allowed transmit power level on the RACH is also an essential piece of information to be transferred from the system so that it can be verified that the transmit power available and the reliability of the transfer are sufficient, and so that power consumption at the mobile station can be minimized.
Furthermore, the maximum packet size allowed on the RACH is also an important piece of information transferred from the system in order to prevent a data transfer failure due to a packet size too large.